Effect of Added Quenchers in Organic Scintillator Solutions: Organometallics

Quenching constants (γ_s and γ_x) are given for a variety of perphenyl metals (Si, Ge, Sn, Pb, Sb, and Hg) and for mercury dimethyl acting on benzene and cyclohexane solvents and p‐terphenyl and DPA scintillators. Specific rates are calculated for the quenching processes and are found to break into the following classes: <7×10⁹ M^—1 sec^—1 corresponding to spin‐perturbation‐induced quenching (with a probability factor less than unity, according to Umberger—LaMer calculations) and ${\text{≧3×10}}^{10}$ M^—1 sec^—1 (exceeding such calculated values), corresponding to highly favored excitation‐transfer processes. The latter high values can be explained in terms of the domain theory of liquid scintillator solutions. Specific rates in the intermediate range for GePh₄, and perhaps for SiPh₄, acting on solvents are consistent with resistance to high‐energy irradiation and with the attendant probability that long‐lived excited states of such quenchers actually transfer excitation to the scintillator. Anomalously high quenching constants and specific rates for quenching (as well as an anomalous relationship between quenching constants for aerated and de‐aerated solutions) in the case of cyclohexane suggest that a proper view of energy deposition should be an initial nonlocalized excitation of the system, followed by competitive localization of excitation in one of several particular species: i.e., cyclohexane (which may immediately decompose if an excited state is directly produced), quencher, or scintillator.